A secondary battery is a battery which is able to provide electricity by converting a loss in chemical energy into electrical energy; moreover, it is a battery which is able to store (during charge) chemical energy by converting electrical energy into chemical energy by passing an electrical current in a direction that is opposite to the discharge direction. Among secondary batteries, lithium ion batteries have higher energy density, so that they are widely used as a power source for notebook personal computers, cellular phones, etc.
In a lithium secondary battery using graphite (C6) as the negative electrode active material, the reaction described by the following formula (I) proceeds at the negative electrode upon discharge:C6Li→C6+Li++e−  (I)
An electron produced by the formula (I) passes through an external circuit, works by an external load, and then reaches the positive electrode. At the same time, a lithium ion (Li+) produced by the formula (I) is transferred through the electrolyte sandwiched between the negative and positive electrodes from the negative electrode side to the positive electrode side by electro-osmosis.
When lithium cobaltate (Li0.4CoO2) is used as a positive electrode active material, a reaction described by the following formula (II) proceeds at the positive electrode upon discharge:Li0.4CoO2+0.6Li++0.6e−→LiCoO2  (II)
Upon charging the battery, reactions which are reverse to the reactions described by the above formulae (I) and (II) proceed at the negative and positive electrodes. The graphite material in which lithium was intercalated (C6Li) becomes reusable at the negative electrode, while lithium cobaltate (Li0.4CoO2) is regenerated at the positive electrode. Because of this, discharge becomes possible again.
Among lithium secondary batteries, a lithium secondary battery all-solidified by using a solid electrolyte as the electrolyte, uses no combustible organic solvent in the battery; therefore, it is considered to be safe, able to simplify the device and excellent in production cost and productivity. A sulfide solid electrolyte is known as a solid electrolyte material used for such a lithium secondary battery.
Many sulfide solid electrolyte-related techniques have been developed so far. A technique is disclosed in Non-patent Literature 1, relating to a sulfide solid electrolyte which has an amorphous structure and is represented by the following chemical formula: 75{(1−x)Li2O.xLi2S}.25P2S5.